Editing Olbers's paradox (section)

{{short description|Argument in astrophysics against the theory of an unchanging universe}}

{{use dmy dates |date=January 2023}}

{{distinguish|Oberth paradox}}

[[file:Olbers' Paradox - All Points.gif|thumb|As more distant stars are revealed in this animation depicting an infinite, homogeneous, and static universe, they fill the gaps between closer stars. Olbers's paradox says that because the night sky is dark, at least one of these three assumptions must be false.|alt=In this animation depicting an infinite and homogeneous sky, successively more distant stars are revealed in each frame. As the animation progresses, the more distant stars fill the gaps between closer stars in the field of view. Eventually, the entire image is as bright as a single star.]]

'''Olbers's<!-- Do not remove an extra "s" - see [[MOS:POSS]] --> paradox''', also known as the  '''dark night paradox''' or '''Olbers and Cheseaux's paradox''', is an argument in [[astrophysics]] and [[physical cosmology]] that says the [[darkness]] of the [[night sky]] conflicts with the assumption of an infinite and eternal [[static universe]].  In the hypothetical case that the universe is static, [[homogeneous]] at a large scale, and populated by an infinite number of [[star]]s, any line of sight from [[Earth]] must end at the surface of a star and hence the night sky should be completely illuminated and very bright. This contradicts the observed darkness and non-uniformity of the night sky.<ref name="NYT-20150803">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=The Flip Side of Optimism About Life on Other Planets |url=https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html |date=August 3, 2015 |work=[[The New York Times]] |access-date=October 29, 2015 }}</ref> 

The darkness of the night sky is one piece of evidence for a dynamic universe, such as the [[Lambda-CDM model|Big Bang model]]. That model explains the observed darkness by invoking [[expansion of the universe]], which increases the [[wavelength]] of [[visible light]] originating from the Big Bang to [[microwave]] scale via a process known as [[redshift]]. The resulting [[Microwave background|microwave radiation background]] has wavelengths much longer (millimeters instead of nanometers), which appear dark to the naked eye. Although he was not the first to describe it, the paradox is popularly named after the German astronomer [[Heinrich Wilhelm Matthias Olbers|Heinrich Wilhelm Olbers]] (1758–1840).